Smith County, TN Insulation Payback
Smith County attic insulation, by the numbers
Smith County, Tennessee is zone 4 under DOE's climate map (Mixed-Humid), which means a genuine mix of cold winters and humid summers, with neither one dominating the bill. An uninsulated attic there should be built up to R-60. For an attic around 1,000 square feet, closing the gap from R-2 to R-60 costs around $3,480 and saves an estimated $1,252 a year — payback in roughly 2.8 years. Topping up the attic pays for itself fast here.
Where the savings come from
Split it out and it's roughly $1,018 a year on the heating side, $234 on cooling. By estimated insulation savings, Smith County lands in the 35th percentile nationally. Zone 4 alone covers 825 counties in this dataset, and Smith County comes in 679th by savings. Tennessee averages $1,258/year across the counties tracked here; Smith County comes in 0% higher.
Smith County's actual weather, not a national average
Smith County's climate figures — 3,807 heating and 1,457 cooling degree days a year — are read from CARTHAGE, TN US, 1.8 miles away, NOAA's nearest 1991-2020 normals station. That combined heating-and-cooling load ranks Smith County 2121st nationally by total degree days — the 33th percentile. One local detail the R-value alone won't tell you: Smith County sits in moisture regime A (humid), which can affect the vapor-barrier choice.
The smaller top-up scenario
If Smith County's attic already has 3-4 inches of old insulation (about R-11) rather than bare joists, the recommended target drops to R-49. That smaller top-up costs around $2,280 and saves an estimated $183/year — payback about 12.5 years. Call this borderline rather than a clear win.
Getting it done in Smith County
Actual quotes will vary by attic access, existing debris, and whether the job needs air sealing first. Treat the $3,480 figure above as a planning number for Smith County, not a bid - contractors typically charge a $600 minimum regardless of job size.
Where these figures come from
Smith County's R-60 target and its degree-day figures both come from named federal sources - see the methodology guide for exactly which ones. The guide to what R-value actually means explains why R-60 is the target here specifically.